The exemplary embodiments are directed to a method and system for employing a control algorithm to converge desired colorimetric color space targets by adjusting mass targets of printing systems in an iterative fashion.
To allow image output from a color printer, a color copier, etc., color conversion needs to be performed beforehand. For example, in a color copier, it is general procedure that a document is read by a line sensor or the like and reading results as output as additive mixture signals of RGB (red, green, and blue). On the other hand, image outputting is performed based on subtractive mixture signals of CMYK (cyan, magenta, yellow, and black). Therefore, color conversion processing in a color copier is performed with an assumption that particular input/output sections such as the above-mentioned ones are used. That is, a color conversion circuit is so adjusted that a test pattern (an original) and its copy resemble each other to as high a degree as possible.
There exists a wide variety of color input/output media and accordingly there are a variety of image data formats. Because image data are exchanged through a network or the like, it is necessary to perform color conversion processing on various kinds of image data. The typical color spaces used in personal computer are, e.g., RGB, HSL, HSB, XYZ, L*a*b* etc., color spaces that are defined from the RGB space by rearranging calculations, and the CMYK space that use recording colors themselves. With the use of CMYK signals, printed colors may be different if different sets of colorants are used. As such, CMYK signals are device-dependent signals. To use device-dependent signals in another device, it is necessary to specify a corresponding relationship (color matching) between the device-dependent signals and device-independent signals. “Device-independent signals” means signals that can be transformed, by known definitive equations, into signals on a colorimetric color space (CIE XYZ, l*a*b*, etc.).
As indicated above, even with the use of the same CMYK signals sent to multiple printing devices, printed colors may be different if different sets of colorants are used. In other words, because CMYK signals are device-dependent, the actual available color gamut (the subset of colors that can be accurately represented by a given output device) depends not only on the function of how much toner is present in each pixel (the smallest area of print that can be independently colored), but is also a function of the process and media. Internal to the process, the developed mass per unit area on a photoreceptor of a printing device is controlled using common set point targets using, e.g., level 2 control loops. Both level 1 and level 2 control loops are known in the art. See U.S. Pat. No. 5,950,040; U.S. Pat. No. 5,754,918, which are hereby incorporated by reference. The developed mass per unit area (“DMA”) depends on the process parameters of each of the color separation printed by the printing device on a media. The process parameters include, for example, photoreceptor voltages (charged and/or discharged voltages), donor and/or magnetic roll voltages, toner concentration, etc. The level 2 process control loops and level 1 process control loops or toner concentration loops generate these process parameters on a real time basis by measuring color patches printed in the inter-document zone with sensors during image printing. However, this process may still give rise to differences in color gamuts across multiple machines. For example, FIG. 1 shows gamut differences across multiple related art machines simulated using a virtual printer model with 9% toner content, represented by solid portion of the graphical depiction and 14% toner content represented by mesh portion of the graphical depiction.